Existing mass spectrometers have an ion source that produces ions of a sample material, and also ions of an internal standard that is mixed with the sample material. These ions are then processed by a mass analyzer which includes a mass detector. Some existing ion sources produce ions using a technique known as electron ionization (EI). A different type of ion source produces ions using a technique known as chemical ionization (CI). In many applications, the mass spectrometer is configured to operate under conditions of higher sensitivity than what is required for accurate qualitative and quantitative analysis. In other words, the quantity of ions produced in the ion volume is much more than what is required for a detection limit of the analysis at hand. When the mass spectrometer is operated in this manner, the instrument can suffer a faster contamination and decay rate. The contamination can cause the sensitivity, tuning and accuracy of mass identification to change over time. As a result, the quality of the analytical data can vary from sample to sample within an analytical batch of samples. Additionally, more frequent calibration and tuning is needed, and more frequent maintenance is needed to clean the ion source and other components of the mass spectrometer, all of which are undesirable.
Several approaches have been used to increase the operational lifetime of the ion source at the expense of giving up some instrument sensitivity. For example, a split mode injection or a sample extract dilution may be utilized. Although these approaches have been generally adequate for their intended purposes, they have not been satisfactory in all respects. As one example, surface active analytes may tail excessively as the chromatographic integrity diminishes, which might otherwise be masked by higher level of analytes. As another example, thermally labile components may react in the hot ion volume of the mass spectrometer or injection port of the gas chromatograph thereby generating poor responses relative to their internal standard, which might also benefit from higher levels of analyte. This is the case with certain routine analytical protocols such as EPA (Environmental Protection Agency) Method 8270, where such adverse performance precludes further analysis.